High solids ratio solid rocket motor propellant grains and method of construction thereof

ABSTRACT

A high acceleration high performance solid rocket motor grain such as for a ballistic defense missile or rocket assisted projectile comprises a propellant material which includes a highly plasticized binder so that the grain has a solids ratio equal to at least about 95 percent. In order that the grain with such a solids ratio may have adequate strength and withstand high acceleration forces, a reticulated structure is embedded therein. A method of constructing a rocket motor having such a grain is also disclosed.

The present invention relates to the construction of solid rocket motorpropellant grains. More particularly, the present invention relates tothe construction of such propellant grains with reticulated structuresembedded therein.

It has been suggested in U.S. Pat. No. 3,191,535 to Mulloy to prepare asolid propellant which consists essentially of a cellular fuel elementhaving uniform interconnecting spherical voids of a metal or metalalloy, and a propellant material filling the voids.

It has also been suggested in U.S. Pat. Nos. 3,616,841 and 3,946,039 toWalz that form retaining reticulated structures of metal or the like maybe used as solid propellant reinforcement and burning rate modifiers.These Walz patents, which are hereby incorporated herein by referenceand made a part of this specification, describe methods for producingsuch a reticulated structure by using as a pattern a self-supportingreticulated polyurethane or organic foam formed of ligaments to providea wide range of pore sizes, varying from 3 to 125 pores per linear inch,and the finished foam material is characterized as having ligamentswhich are continuous, gas-free or of low porosity, and of integralconstruction. The Walz patents further disclose that the resulting foamstructure may be trimmed or shaped or chemically or electrically etched,and that various modifications (unspecified), both chemical andphysical, can be carried out with the reticulated foam. Such a structureis generally known to be machinable.

The Walz patents do not disclose or suggest, however, how to construct acentrally perforated propellant grain with a reticulated structureembedded therein. It is therefore an object of the present invention toprovide a method of constructing a rocket motor with a reticulatedstructure embedded in a centrally perforated propellant grain thereof.

For solid rocket propellant grains without reticulated structuresembedded therein, solids ratios of up to about 92 per cent have beenachieved with the use of hydroxy terminated polybutadiene (HTPB)binders. By "solids ratio", as that term is used in this specificationand the claims, is meant the ratio by weight of propellant grainmaterials which are solid at the processing temperature such as aluminumpowder and other solid fuel particles, reticulated structures, ammoniumperchlorate, and solid burn rate catalysts to the total weight ofmaterial in the propellant grain. The solids ratio is a measure of theenergy available in the grain to produce thrust. Prior to being cast ina rocket motor case, the propellant material is formulated and mixed andthen flowed into the case. As the solids ratio of mixed propellantmaterial with low plasticizer content binders increases above 92percent, it becomes too viscous for practical processing. Although acast propellant material which has a solids ratio of 95 percent orhigher may be provided if a highly plasticized binder is used, such apropellant material may not have sufficient binding material, i.e.,polymer to provide sufficient strength. It is therefore another objectof the present invention to provide a solid rocket motor propellantgrain which has an increased solids ratio in the range of 95 percent orhigher for increased performance yet also has sufficient strength foruse.

Ballistic defense missiles, rocket assisted projectiles, and the likerequire high accelerations often in the order of 100 gs or greater inorder to achieve their desired objectives. However, such highaccelerations may not be withstood by the propellant grains unless theyare adequately reinforced. It is yet another object of the presentinvention to provide such a propellant grain which can withstand suchhigh acceleration forces.

It is still another object of the present invention to provide such apropellant grain which also has increased range.

For rockets which are spin stabilized, there is a tendency forcentrifugal force to cause diffusion of solid fuel particles outwardlytoward the case with resulting burn rate augmentation effects which itis desirable to avoid. Such effects have been reduced by incurringperformance penalties such as reduced spin. It is thus another object ofthe present invention to provide a propellant grain wherein theresulting burn rate augmentation effects from high spin velocities areminimized but without incurring performance penalties.

The above and other objects, features, and advantages of this inventionwill be apparent in the following detailed description of the preferredembodiments thereof which is to be read in connection with theaccompanying drawings.

In the Drawings:

FIG. 1 is a perspective view of a propellant grain portion with areticulated structure embedded therein but with a portion of thereticulated structure left unfilled with propellant mass to betterillustrate the reticulated structure of the present invention;

FIG. 2 is a stress-strain diagram for propellants reinforced withreticulated structures and one which is unreinforced;

FIG. 3 is a schematic view illustrating the machining of a perforationaxially through the center of a reticulated structure in accordance withthe present invention;

FIG. 4 is a perspective view illustrating a case with a reticulatedstructure, machined as illustrated in FIG. 3, inserted therein and withportions of the case and reticulated structure cut away; and

FIG. 5 is a section view of the case of FIG. 4 in which the reticulatedstructure of FIG. 4 and a core have been inserted so that the case maybe filled with propellant material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is illustrated at 10 a solid propellant grainportion which includes an homogeneous mass 12 of propellant materialincluding a suitable oxidant in which is embedded a reticulatedstructure 14 of combustible or non-combustible material such as, forexample, but not limited to, aluminum, boron, magnesium, beryllium,copper, alumina, carbon, polyurethane, graphite, and zinc. If it is notnecessary that the propellant be smokeless, it is usually preferred thatthe reticulated structure 14 be composed of combustible material suchas, for example, but not limited to, aluminum, boron, beryllium, orcopper so that it will also burn as the homogeneous mass of propellantburns to provide increased energy. However, if it is desired that thepropellant be smokeless, it is preferred that the reticulated structure14 be composed of carbon, graphite or a non-combustible material suchas, for example, boron nitride, silicon carbide, alumina, or a highmelting point metal such as titanium or a zirconium-titanium alloy.Although carbon and graphite are combustible under some conditions suchas if the propellant mass is oxygen rich, they produce non-smokeproducing carbon dioxide when they burn. For the purposes of thisspecification and the claims, the term "homogeneous mass of propellantmaterial" refers to a propellant material of uniform structure orcomposition and is meant to include propellant mixtures commonly knownas composite propellants as well as homogeneous propellant compoundscommonly known as homogeneous propellants. A portion (the upper portionin FIG. 1) of the grain 10 has been left unfilled with propellantmaterial in order to better illustrate the reticulated structure 14which is an isotropic structure similar to the structures described inthe Walz patents previously referred to. Such a structure is composed ofa multitude of ligaments which are of generally uniform dimension andinterconnected with each other to form voids which are open to eachother so that they may be filled with propellant or other material. Forthe purposes of this specification and the claims, the term "reticulatedstructure" is meant to refer to a structure which is composed of amultitude of ligaments interconnected with each other to form voidswhich are open to each other and includes such a structure as describedin the Walz patents.

FIG. 2 shows a stress-strain diagram for an unreinforced propellant at22 and the propellant reinforced with 10 pores per linear inch and 20pores per linear inch respectively reticulated aluminum similar to thatdescribed in the aforesaid patents to Walz at 24 and 26 respectively.The propellant in FIG. 2 is composed of the following by weight percent:

    ______________________________________                                        Hydroxy terminated Polybutadiene (binder)                                                              9.990                                                Catocene (burning rate catalyst)                                                                       4.000                                                Aluminum powder (fuel)   18.000                                               Ammonium perchlorate (oxidizer)                                                                        68.000                                               Triphenyl bismuth (cure catalyst)                                                                      0.010                                                ______________________________________                                    

The reinforced grains 24 and 26 were composed of the aforementionedpropellant with a portion of the aluminum powder replaced by reticulatedaluminum so that the reinforced grains contained 13.491 percent aluminumpowder and 4.509 percent reticulated aluminum. The tests resulting inthe stress-strain diagram were conducted at a temperature of 77° F., across head speed of 2 inch/minute, and a specimen gage length of 1.25inch. The ability of propellant grains to withstand high accelerationsis related to the modulus of elasticity thereof, i.e., the greater themodulus of elasticity, the better a propellant grain is able towithstand high accelerations. As FIG. 2 indicates, the modulus ofelasticity of a propellant grain as well as the stress which it canwithstand may be increased greatly by embedding therein a reticulatedstructure. Such an increase in modulus of elasticity may be on the orderof 4 times or more.

Referring to FIGS. 3 to 5, there is illustrated a method of constructinga rocket motor section with a reticulated structure embedded in thepropellant grain. In accordance with the method, the reticulatedstructure, similar to reticulated structure 14 in FIG. 1 and illustratedat 30 in FIG. 3, is machined to a predetermined shape. This includesmachining such as by milling, as illustrated in FIG. 3, by use of acutter member 32 having a blade 34 and rotating as illustrated at 37, aperforation, illustrated at 36, axially (i.e., in the direction 40)through the center of the structure 30. By "axially" is meant adirection, illustrated at 40 in FIGS. 3 and 4, parallel to thelongitudinal axis 42 of a rocket motor when the structure 30 is properlypositioned therein. In order to provide a predetermined propellant grainshape such as, for example, the illustrated type of star shape, thecutter member 32 is successively moved radially of the structure 30(i.e., in the directions illustrated at 39 in FIG. 3) for successivecuts in the axial direction 40 thereof to form a series of parallelinterconnecting perforations defining the desired grain shape.

Referring to FIG. 4, a tube shaped reticulated structure 41, machined inaccordance with the method illustrated in FIG. 3, is then inserted in amotor case 38.

Referring to FIG. 5, the case 38, with the reticulated structure 41contained therein is then positioned within a casting assemblyillustrated generally at 50 for casting of propellant. The case 38 issealingly engaged by means of rubber gaskets 52 or other suitablegaskets at both ends between alignment rings 54 and 56 composed of orcoated with a synthetic resin polymer sold under the trademark Teflon.These alignment rings are clampingly held to the case 38 and reticulatedstructure 41 by respective housing members 58 and 60 and a pair ofthreaded bolts 62 extending between the housing members 58 and 60.Alignment ring 56 has a radially inner star-shaped portion, asillustrated at 64, having perhaps eight tips 65 spaced circumferentiallythereof and extending radially inwardly to engage and center the core66, which is shaped to conform to the shape of the internal perforation72 of reticulated structure 41, and to permit excess propellant toescape between the tips during casting. A core puller 68 is attached bymeans of threaded screw 70 to one end of the core 66. In order to castthe propellant in the case 38, the core 66 is partially inserted withinperforation 72 of the reticulated structure 41 over substantially theentire length of the reticulated structure 41 but leaving an opening 74at the forward tapered end 76 between the reticulated structure 41 andthe core 66. The propellant is then cast under pressure in the directionillustrated by arrows 78 through the opening 74 to fill the case 38including voids in the reticulated structure 41, care being taken toprevent air pockets (voids) from forming during the filling process.Then the core 66 is further inserted until the core puller 68 is seatedagainst the housing member 60 and the opening 74 is closed, and thepropellant is allowed to cure at a suitable temperature of typically 145degrees F. The core puller 68 is then used to remove the core 66 whilethe casting assembly 50 is restrained by an hydraulic clamp (not shown)after which the casting assembly 50 is disassembled to remove the rocketmotor section.

In accordance with an alternative embodiment of this invention, the casemay if desired be first filled with propellant and then the reticulatedstructure and core inserted into the propellant filled case.

The core 66 is preferably composed of a non-spark inducing and lowlocalized heat generating material such as a synthetic resin polymersold under the trademark Teflon, polyethylene, polystyrene, or a highmelting point wax to prevent sparking or high localized heat generationas the core 66 is removed. By "low localized heat generating material"is meant a material which does not generate sufficient localized heatupon normal frictional contact with the reticulated structure to ignitethe propellant.

If it is desired that the reticulated structure 41 and core 66 each beconstructed of a metal or other material wherein sparks may be induced,the core 66 should be maintained over its length in a spaced relationfrom the reticulated structure 41, for example, by means of suitablysizing the core and reticulated structure, to prevent spark inducingcontact between the core and the propellant impregnated reticulatedstructure as the core is removed therefrom. If the core 66 is composedof a metal, it is preferably coated with a wax or a synthetic polymersold under the trademark Teflon to facilitate its removal easily fromthe propellant grain after it has been cast.

It is desirable to utilize propellant grains of high solids ratio, i.e.,ratios in the range of 95 percent and greater, for high performanceapplications due to their increased energy content. However, apropellant composition which contains a low plasticizer content binder,i.e., a binder having less than about 20 percent plasticizer, may becometoo viscous for practical processing as the solids ratio increases aboveabout 92 percent.

A binder usually contains a polymer, a curing agent, and a liquidplasticizer, except that thermoplastic and thermosetting binders do notcontain curing agents. A typical low plasticizer content binder maycontain, by weight, about 10 to 20 percent liquid plasticizer such as,for example, dioctyl adipate, dioctyl sebacate, dioctyl phthalate, ornon-functional polybutadiene, about 7% curing agent such asdi-isocyanate, and about 73 to 83% polymer such as HTPB (hydroxyterminated polybutadiene). Another example of a low plasticizer contentbinder may contain, by weight, about 10 to 20 percent liquid plasticizersuch as those named above and about 80 to 90 percent thermoplasticelastomer polymer such as, for example, a co-polymer of polystyrene andbutadiene, a multi-block polyurethane, a polyolefin blend, polyamide, ora polypropylene-EPDM blend. A liquid plasticizer is defined, for thepurpose of this specification and the claims, as a material that has aviscosity less than 200 centipoise and whose chemical formula does nothave reactive sites.

In order to provide a solid rocket motor propellant grain which is nottoo viscous for practical processing and which has an increased solidsratio in the range of 95 percent or higher in accordance with thepresent invention, the propellant material therefor is composed of ahighly plasticized binder. For the purposes of this specification andthe claims, a "highly plasticized binder" is defined as a binder whichhas a liquid plasticizer content equal to at least about 50 percent, byweight, of the binder. The plasticizer content is preferably less thanabout 30 percent, by weight, of the binder. However, such a propellantmaterial may not contain sufficient binding material, i.e., polymer, toprovide sufficient strength and also the ability to withstand highacceleration loads. Therefore, further in accordance with the presentinvention, a reticulated structure 41 is embedded in the highlyplasticized propellant material to add sufficient increased strength asillustrated in FIG. 2 to the resulting grain and also to provide theability to withstand acceleration loads in the range of 100 gs, i.e.,100 times the force of gravity or more. In addition, the reticulatedstructure 41 may be preferably composed of oxidizable material in orderto further increase the solids ratio by perhaps 0.5 percent.

An example of a propellant grain embodying the present invention is asfollows wherein the percentages are by weight:

    ______________________________________                                        Aluminum (in reticulated structure)                                                                 8.2%                                                    Aluminum powder       13.8%                                                   Ammonium perchlorate  72.0%                                                   Iron oxide            1.0%                                                    Triphenyl bismuth     0.01%                                                   Binder (highly plasticized)                                                   dioctyl adipate liquid plasticizer                                                                  2.5%                                                    di-isocyanate curing agent                                                                          0.3%                                                    HTPB polymer          2.19%                                                   ______________________________________                                    

Such a propellant grain has 95.01 percent solids for increasedperformance yet is sufficiently fluid, before casting, for processing,and the reticulated structure is provided so that the grain may haveadequate strength and withstand high acceleration loads.

It may also be desirable to provide such a propellant grain which alsohas a high web fraction, i.e., a web fraction greater than 0.7, in orderto provide higher loading density for increased range to a rocket. By"web fraction", for the purposes of this specification and the claims,is meant the difference between the outside radius 44 and bore radius 46of a grain divided by the outside radius 44 thereof. However, for suchhigh web fraction grains, there is an increased probability of cure andthermal shrinkage leading to propellant cracking and/orpropellant-liner-insulation failure. Therefore, not only may a grainreinforced with a reticulated structure 30 be provided with a highsolids ratio for high performance and acceleration as previouslydescribed but such a grain may also be with a high web fraction forincreased range with the reticulated structure 41 used to also minimizesuch shrinkage. Thus, a reticulated structure reinforced grain inaccordance with the present invention may be provided for both increasedacceleration and increased range.

It is to be understood that the invention is by no means limited to thespecific embodiments which have been illustrated and described herein,and that various modifications thereof may indeed be made which comewithin the scope of the present invention as defined by the appendedclaims.

What is claimed is:
 1. A method of constructing a rocket motor with thepropellant grain thereof including a reticulated structure embeddedtherein comprises the steps of:a. forming a reticulated structure tohave a predetermined propellant grain shape; b. inserting the formedreticulated structure in a motor case; c. preparing a propellantmaterial which includes a highly plasticized binder such that the grainhas a solids ratio equal to at least about 95 percent; d. at leastpartially filling the case including voids in the reticulated structurewith the propellant material; and e. casting the propellant material inthe case.
 2. A method according to claim 1 wherein the step of formingthe reticulated structure includes machining a perforation axiallythrough the center thereof, and the method further comprises providing acore which has a shape which conforms to the shape of the perforationand inserting the core in the perforation prior to filling the case withpropellant material, and removing the core after casting of thepropellant material.
 3. A method according to claim 2 wherein thereticulated structure and core are each composed of a metal, the methodfurther comprises spacing the core from the reticulated structure toprevent spark inducing and high localized heat generating metal to metalcontact as the core is removed.
 4. A method according to claim 2 furthercomprises selecting the core to be composed of a non-metallic lowlocalized heat generating material.
 5. A method according to claim 2wherein step b further comprises inserting the case with the reticulatedstructure contained therein within a casting assembly, sealinglyengaging the case at both ends thereof, partially inserting the core inthe perforation such than an opening is left at one end through which isflowed the propellant material, centering the core, flowing thepropellant material through the opening to fill the case with propellantmaterial, and further inserting the core until the opening is closedafter the case has been filed with propellant material to allow castingof the propellant material.
 6. A method according to claim 5 furthercomprises selecting the core to be composed of a non-metallic lowlocalized heat generating material.
 7. A method according to claim 1further comprises selecting the reticulated structure to be composed ofa combustible material.
 8. A method according to claim 7 furthercomprises sizing the grain to have a web fraction greater than 0.7.
 9. Amethod according to claim 1 further comprises sizing the grain to have aweb fraction greater than 0.7.
 10. A method according to claim 1 furthercomprises selecting the binder to have a liquid plasticizer selectedfrom the group consisting of dioctyl adipate, dioctyl sebacate, dioctylphthalate, and non-functional polybutadiene.
 11. A method according toclaim 10 further comprises selecting the binder to have a HTPB polymerand a di-isocyanate curing agent.